Photolithography is a process used in semiconductor integrated circuit device fabrication to produce device structures on semiconductor or other substrates. Distortions of device structures are becoming evident in view of the shrinking of the dimensions of the device structures as compared to the radiation wavelengths used during photolithography. One source of distortion is due to light scattered or otherwise affected by adjacent structures. Distortion in size and shape of the projected image exhibited by this phenomenon is called proximity effect.
In optical proximity correction (OPC), a resolution enhancement technique using scattering bars has been introduced to counter proximity effects and to reduce distortion. Scattering bars are sub-resolution assist features (SRAF) that are placed on a mask (e.g., reticle or photo-mask) adjacent to isolated features and/or semi-isolated features. Isolated and semi-isolated design features, such as metal lines, trenches, or gate polysilicon, are generally exposed and/or printed on the substrate at a feature size significantly different from the same design feature surround by other nearby features. This phenomenon is known as an isolated/dense proximity effect. The use of scattering bars enables these isolated and/or semi-isolated design features to form more like dense features. In this manner, the usable resolution of an imaging system may be extended without decreasing the radiation wavelength or increasing a numerical aperture of the imaging tool, although such processes can be used for additional benefit.
Conventional scattering bars are narrow lines placed adjacent to existing design features. The scattering bars are parallel with the isolated feature, often with scattering bars placed on either side of an isolated feature. These types of scattering bars are commonly called edge scattering bars. Where there are semi-isolated features, for example two parallel lines spaced apart from one another, a center scattering bar is typically placed in parallel with and between the semi-isolated features. However, when the semi-isolated features are beyond a certain distance apart, the center scattering bar becomes spaced too far from the semi-isolated features and the benefit of using scattering bars significantly diminishes.